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Microsoft released a removal tool for the MSBlast worm on Monday after Internet service providers complained that home users' PCs infected with the malicious program are still causing network congestion.

The MSBlast worm, also commonly called the Blaster worm, started spreading last August and is believed to have spread to hundreds of thousands of systems. While most corporations have cleaned up the worm, Microsoft has found that a large number of home users are still unknowingly infected, the software giant said in a statement.

"For many users in this situation, there is little indication that they are infected other than possible performance degradation," Microsoft said. "And those infected are still actively transmitting the worm, causing Internet congestion in the process."

Worms--programs that spread between vulnerable computers over a network--have been the bane of Microsoft since Code Red and Nimda hit computers running the company's operating system in summer 2001. Nimda, which hit computers a week after the Sept. 11, 2001, terrorist attacks, put Microsoft on the path toward making security its No. 1 priority. Four months later, Microsoft announced its Trustworthy Computing Initiative to do just that.

However, Microsoft developers have had their work cut out for them. Despite a Windows work stoppage of more than two months while programmers reviewed the code, security flaws have continued to crop up and be exploited by malicious worm writers.

A year ago, the Slammer worm spread quickly among Windows servers and devices that had Microsoft's SQL database software installed. And the MSBlast worm abused a 23-day-old vulnerability, spreading in August and September.

Microsoft has since set up a site called Protect your PC to help home users lock down their computers, and it posted a $250,000 bounty for the writers of the MSBlast worm and the SoBig.F virus.

Microsoft's aim in releasing the latest tool is to reduce the amount of traffic being borne by ISPs by cleaning up a significant number of home computers.